1. Field of the Invention
Example embodiments of the present invention relate to an off-axis projection optical system and an extreme ultraviolet (EUV) lithography apparatus using the same.
2. Description of the Related Art
In a photolithography process of a semiconductor manufacturing process, a light exposure technology used to realize a direct writing resolution of less than 100 nm may involve the use of a light exposure wavelength within the EUV band. In EUV lithography technology, extreme ultraviolet light having a very short wavelength of less than 100 nm, for example, a wavelength of about 13.5 nm, may be used.
Because most materials absorb light in the EUV band, it may be difficult and/or impossible to use a refractive optical device for EUV light. Therefore, a reflection mask may be required for the light exposure technology using the EUV light. In order for the EUV light reflected from the reflection mask to travel toward a wafer, a conventional projection optical system having a plurality of reflection mirrors may be used and/or required. The EUV light may be radiated to the reflection mask installed in a chamber. The EUV light reflected from the reflection mask may be incident on the wafer after being reflected by the plurality of reflection mirrors of the projection optical system, thereby forming a pattern corresponding to the mask on the wafer.
As described above, conventional EUV light lithography uses and/or requires a projection optical system having a plurality of reflection mirrors.
FIG. 1 schematically illustrates a conventional on-axis projection optical system, which may be used in a light exposure apparatus for performing a conventional EUV lithography process.
Referring to FIG. 1, EUV light 4 radiated from a EUV source onto a mask 1 may be reflected by the mask 1 and then incident on a light exposure apparatus 2.
First and second mirrors 3 and 5 for reflecting the EUV light 4 toward an image plane where a wafer 6 may be disposed may be included in the light exposure apparatus 2. First and second center holes 3a and 5a through which the EUV light 4 passes may be formed in the center of the first and second mirrors 3 and 5, respectively. The second mirror 5 may be disposed near the mask 1 while the first mirror 3 may be disposed relatively far from the mask 1.
The EUV light 4, which may be reflected from the mask 1 and incident on the second mirror 5 after passing through the second center hole 5a formed in the second mirror 5 is divergent. The EUV light 4 incident on the first mirror 3 may be reflected to the second mirror 5, which may reflect the EUV light 4 to form a convergent beam. Then, the EUV light 4 may be radiated to the wafer 6 onto the image plane after passing through the first center hole 3a formed in the first mirror 3.
In a conventional on-axis projection optical system, because the center holes 3a and 5a may be formed in the first and second mirrors 3 and 5, respectively, stray light 4′ traveling directly from the mask 1 toward the wafer 6 through the first and second center holes 3a and 5a of the respective first and second mirrors 3 and 5 may exist.
Therefore, a conventional light exposure apparatus 2 using a typical on-axis projection optical system requires a shielding member 7. An example of the shielding member 7 is illustrated in FIG. 2. The shielding member may reduce and/or prevent stray light from traveling directly toward the wafer 6. The shielding member 7 may be disposed at a location between the first and second mirrors 3 and 5 to block a central portion of the EUV light 4. That is, the stray light 4′ traveling toward the first center hole 3a through the second center hole 5a of the second mirror 5 may be partially or completely blocked by the shielding member 7.
The shielding member 7 may be supported by a narrow strip-shaped support 8 so the shielding member 7 may block the central portion of the EUV light 4. Referring to FIG. 2, the support 8 for supporting the shielding member 7 may be cross-shaped, for example.
A drawback of conventional on-axis projection optical systems is that the shielding member 7 for blocking the stray light 4′ is necessary. Further, the shielding member 7 may cause diffraction, scattering and flare, for example.